The present application claims priority to Japanese Application No. P2001-139438 filed May 10, 2001, which application is/are incorporated herein by reference to the extent permitted by law.
This invention relates to a magnetic memory device which is used as a memory device for storing information.
In recent years, information communication equipments, particularly small equipments for personal use such as portable terminal equipments, have exhibited remarkable popularization, and it is demanded for component devices for such equipments such as memories and logic circuits to have further augmented performances in terms of the circuit integration, operation speed, power saving and so forth. Particularly, increase in density and capacity of a nonvolatile memory is considered to become increasingly significant as a complementary technique for replacing a hard disk apparatus or an optical disk apparatus whose miniaturization is essentially difficult because of the presence of a movable part such as, for example, a head seek mechanism or a disk rotating mechanism.
As a nonvolatile memory, a flash memory for which a semiconductor is used and a FeRAM (Ferro electric Random Access Memory) for which a ferroelectric substance is used are widely known. However, the flash memory has an information writing speed on the order of microseconds and is inferior to a volatile memory such as a DRAM (Dynamic Random Access Memory) or an SRAM (Static Random Access Memory) in that the information writing speed is low. Meanwhile, it is pointed out that the FeRAM has a drawback that the maximum write number thereof is small.
Thus, as a nonvolatile memory which does not have such drawbacks as described above, a magnetic memory device called MRAM (Magnetic Random Access Memory) attracts attention. The MRAM uses a storage element of the giant magnetoresistive (GMR) type or the tunnel magnetoresistive (TMR) type to store information. The MRAM attracts increasing attention as a result of augmentation in performance of a TMR material in recent years (Refer to Naji et al. ISSCC2001).
Here, a principle of operation of the MRAM is described briefly. The MRAM has storage elements (cells) of the magnetoresistive type disposed in a matrix and has conductors (word lines) and read lines (bit lines) extending perpendicularly to each other across the storage elements for recording information into the storage elements such that information is stored selectively into a storage element positioned in an intersecting region between a word line and a bit line. In particular, writing into a storage element is performed by controlling the magnetization direction of the magnetic substance of each storage element using a composite magnetic field generated by supplying current to both of a word line and a bit line. Usually, information of xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d is stored in accordance with the direction of magnetization. On the other hand, readout of information from a storage element is performed by selecting a storage element using such an element as a transistor and extracting the magnetization direction of the magnetic substance of the storage element as a voltage signal through the magnetoresistive effect. As a film configuration for a storage element, a three-layer structure including a ferromagnetic layer, an insulator layer and another ferromagnetic layer, that is, a structure called magnetic tunnel junction (MTJ), has been proposed. Accordingly, if the direction of magnetization of one of the ferromagnetic layers is used as a fixed reference layer and that of the other ferromagnetic layer is used as a recording layer, then since the magnetization direction of the recording layer corresponds as a voltage signal through the tunnel magnetoresistive effect, extraction of information as such a voltage signal can be realized.
Subsequently, selection of a storage element upon writing is described in more detail. Generally, it is known that, if a magnetic field in the direction opposite to the magnetization direction of a ferromagnetic substance is applied in the direction of the easy axis of magnetization of the ferromagnetic substance, then the magnetization direction is reversed to the direction of the applied magnetic field at a critical valuexc2x1Hsw (hereinafter referred to as reversal magnetic field). The value of the reversal magnetic field can be determined from a minimum energy condition. Also it is known that, if a magnetic field is applied not only in the direction of the easy axis of magnetization but also in the direction of the hard axis of magnetization, then the absolute value of the reversal magnetic field decreases. Also this can be determined from a minimum energy condition. In particular, where the magnetic field applied to the direction of the hard axis of magnetization is represented by Hx, the magnetic field Hx and the reversal magnetic field Hy then have a relationship given by Hx(2/3)+Hy(2/3)=Hc(2/3). Hc represents the anisotropic magnetic field of the recording layer. Since this curve forms an asteroid on the Hx-Hy plane as seen in FIG. 1, it is called asteroid curve.
Selection of a storage element can be explained simply using the asteroid. In the MRAM, the magnetization is reversed by a generated magnetic field from a word line to record information. However, since a plurality storage elements are positioned at an equal distance from the word line, if current for generating a magnetic field higher than the reversal magnetic field is supplied to the word line, then the information is recorded similarly to all of the storage elements positioned at the equal distance. However, if current is supplied to a bit line intersecting with a storage element to be selected so that a magnetic field in the direction of the hard axis of magnetization is generated, then the reversal magnetic field at the storage element to be selected decreases. Accordingly, where the reversal magnetic field then is represented by Hc (h) and the reversal magnetic field when the bit line magnetic field is xe2x80x9c0xe2x80x9d is represented by Hc(0), if the word line magnetic field H is set so as to satisfy Hc(h) less than H less than Hc(0), then the information can be selectively recorded only into the storage element to be selected. This is a method of selection of a storage element of the MRAM upon information recording.
The MRAM of such a configuration as described above not only has such characteristics that it is nonvolatile and that it allows non-destructive readout and random accessing, but also has the following characteristics. In particular, since it is simple in structure, higher integration is easy, and since information recording is performed by rotation of the magnetic moment of a storage element of the magnetoresistive type, the maximum write number is great (for example, more than 1016 times) Further, it is estimated that the access time is very short, and it has been confirmed that the MRAM can operate on the order of nanoseconds (for example, 5 ns or less). Furthermore, since the MRAM can be formed only by a wiring step after a MOS (Metal Oxide Semiconductor) preparation step, it has a good matching property. The MRAM is superior to the flash memory particularly in terms of the three points of the maximum write number, random accessing and high speed operation, and is superior to the FeRAM in terms of the process matching property. Besides, since it is expected that both of high integration equivalent to that of the DRAM and high speed operation equivalent to that of the SRAM can be achieved, there is the possibility that the MRAM may be used as a leading storage element for a memory device.
One of subjects to be solved with regard to the MRAM is that the center of the asteroid curve is displaced from the origin on the Hx-Hy plane (hereinafter referred to simply as xe2x80x9coriginxe2x80x9d). More particularly, the asteroid curve is displaced, for example, by approximately 10 Oe (oersted) in the negative direction on the easy axis of magnetization as seen in FIG. 2.
If the center of the asteroid curve is displaced from the origin, then the asymmetry of the asteroid curve makes operation of the MRAM when information of xe2x80x9c0xe2x80x9d is to be written into a storage element and operation of the MRAM when information of xe2x80x9c1xe2x80x9d is to be written into the storage element different from each other. In other words, the current of the word line when xe2x80x9c1xe2x80x9d is to be written and the current of the word line when xe2x80x9c0xe2x80x9d is to be written become different from each other, and as a result, the burden to a pulse current generation circuit for applying current to the word line increases. Further, while the operation margin of a storage element, that is, the difference in the reversal magnetic field between a storage element of an object of writing and an adjacent storage element which is not an object of writing, exhibits its maximum when the reversal field of the non-writing object storage element coincides with a vertex of the asteroid curve, if the center of the asteroid curve is displaced from the origin, then also the operation margin decreases accordingly. From this, it is desirable to make the center of the asteroid curve coincide with the origin.
It is considered that the reason why the center of the asteroid curve is displaced from the origin is that the recording layer of a storage element is influenced by some magnetic field and consequently has an asymmetry in that it is likely to be directed in a particular one direction on the easy axis of magnetization. The influence of the magnetic field in this instance may be, for example, an influence of a generated magnetic field by the fixed reference layer on the recording layer or an influence of a magnetic field which is generated by an influence of the accuracy in film formation (uniformity, surface roughness and so forth of a film) of the insulator between the fixed reference layer and the recording layer. However, such influences cannot be eliminated unless the film thickness and so forth are managed strictly upon film formation of each layer, and they are not problems which can be solved readily if the efficiency, cost and so forth upon manufacture are taken into consideration.
It is an object of the present invention to provide a magnetic memory device wherein an imbalance between variation threshold values for different magnetization directions of storage elements thereof such as a displacement of the center of an asteroid curve from the origin can be corrected readily without modifying the storage elements themselves to allow reduction of the burden to a pulse current generation circuit upon information recording into a storage element and increase of the operation margin when each storage element is selected.
In order to attain the object described above, according to the present invention, there is provided a magnetic memory device, including a plurality of storage elements of the magnetoresistive type capable of storing information making use of a variation of the magnetization direction thereof, and magnetic field application means for applying a bias magnetic field to the storage elements.
With the magnetic memory device, even if the center of the asteroid curve is displaced from the origin, if the magnetic field application means applies a bias magnetic field acting in the opposite direction to the direction of the displacement, then the displacement of the asteroid curve is corrected by the bias magnetic field. Accordingly, only by providing the magnetic field application means for applying a bias magnetic field to the storage elements, an imbalance in variation threshold value of the magnetization direction of the storage elements can be corrected.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.